The weather pattern for this second week in October is looking more and more like what we might expect around Thanksgiving. A very chilly airmass has been building in Canada over the last week or so and with a –NAO in place, that airmass is taking aim at the Great Lakes region and Northeast U.S. this week. Look for below normal temperatures and with these cold temperatures we have two legit snow threats for the mountains, the first on Monday Night and Tuesday and the second coming late in the week. For this post I’m going to focus on the first event on Monday Night and Tuesday where I expect a 3-6” snowfall above 2,500ft with 1-3” between 1,500-2,500ft in Green Mountains from Sugarbush/MRG northward, and in the northern Whites (Cannon, Bretton Woods, Wildcat). Across the Adirondacks where colder temperatures but less precipitation are expected, look for 2-4” of snow above 2,000ft with a coating to 2” above 1,000ft. This will be both a latitude and elevation dependent storm.
A very strong upper level jet (150-200kts at times) has been cranking across the northern tier of the U.S. for the last week and this has been giving the weather models a difficult time with regards to the timing and interaction of different features. The strength of the upper level jet is caused by the large thermal gradient that exists right now between the abnormal early season cold to the north (northern tier of U.S. and in Canada) and milder air to the south (central/southern U.S.) and because of this, I’d expect the weather models to continue having difficulties resolving the interaction and timing of atmospheric features. This is the reason why last week it looked like we could have a big snowstorm on Monday Night, then it looked like all rain as the storm appeared to be headed north of us, and now it appears like we’ll get a moderate snowfall out of it in the higher elevations. It certainly is a pattern where threats exist but details won’t be known until 24-48 hours lead time.

Synoptically, on Sunday Night and Monday a northern stream upper level disturbance will drop out of Canada, cross the upper Great Lakes region, and will be over NY state by late in the day on Monday. This upper level disturbance will have a weak surface area of low pressure as it crosses NY state (probably tracking along the I-90 corridor) but as the upper level trough and associated vorticity digs into the northeast, it will spawn a coastal low, probably just off the MA coastline early Tuesday morning. This low will then track northeastward towards the Canadian Maritimes. Now, this will be a weak coastal low but its important because it’ll cause low level winds to swing around to the northwest on Tuesday…northwest winds in a weak cyclonic flow means the northern Greens and northern Whites will likely see some orographic enhancement on Tuesday.

As for precipitation amounts…the models vary on the amount of QPF (quantitative precipitation forecast) forecast for this event. Some runs have been showing as little as .1” and some are as high as 1”. I’m a little concerned about the lack of a robust surface low but think that mid/upper level support will overcome that. We’ll be in the left front entrance region of the upper level jet which will lead to divergence aloft and as the shortwave trough approaches from the Great Lakes region it will induce rising air ahead of it. Given this, I expect a widespread light to moderate precipitation event with between .25” and .75” of QPF falling…with the lower amounts falling across the Adirondacks and heavier amounts falling from the Green Mountain spine eastward into NH.

Temperatures will be more than cold enough aloft to support snow out of this north of a Glens Falls-Rutland-Lebanon line but the problem will be warmth in the low levels. H85 (~4,500ft) temperatures will range from 0C down near the Killington region to -4C up across northern VT and northern NY. These are marginal for this time of year but luckily most of the precipitation falls late Monday night and Tuesday morning during the coldest time of day. This should allow for dynamic cooling (the process of melting snowflakes uses up latent heat and thus cools the surrounding air) to drop the snow levels down to around 1,500ft or even a bit lower by early Tuesday morning across the Adirondacks and northern VT. In fact, today’s 12z NAM drops snow levels to as low as 1,000ft along an axis from Whiteface to Jay Peak. Further south from central VT to central NH (Killington to Loon area) snow levels will be a bit more tricky. As the coastal low takes shape, an on-shore SE flow develops in the low levels for a few hours on Tuesday morning. This may be enough to cause snow levels to briefly rise in these areas on Tuesday morning before falling again Tuesday afternoon with cold air advection. I could envision snow levels getting down as low as 2,000ft by early Tuesday morning, only to rise back up towards 3,000ft and above with the SE flow later on Tuesday morning, before falling again Tuesday afternoon.

For snowfall totals, all of the above equals a 3-6” snowfall above 2,500ft in the Green Mountains from Sugarbush/MRG northward, and the northern White Mountains (places like Cannon, Bretton Woods, and Wildcat). Any 6” amounts would be at the summits up near 4,000ft. Below 2,500, I’m expecting 1-3” down to 1,500ft in those same areas. The Adirondacks are sort of a wild card here as they will be colder overall but will see less precipitation. I’ll forecast 2-4” in the ‘Dacks above 2,000ft with a coating to 2” possible above 1,000ft. There is a chance of snow accumulations reaching down into the Killington/Pico region in VT and also down into central NH around Loon but if this happens snow accumulation levels will likely remain high near 3,000ft or above. Given that the central region will see the heaviest precipitation, there is a high bust potential if it does tend to lean towards more snow than rain. It wouldn’t take much to get 4-6” of mashed potatoes at the top of Killington Peak above 4K feet with a tight snow line below that…it all depends on the exact track of the low and exact latitude of the 0C isotherm at H85 (4,500ft).

Whew, sorry for the length on this one but there’s a lot to digest in these marginal early season storms. I’m thinking Tuesday might be a decent day for some junkboarding at places like MRG, Stowe, Cannon, Jay, and Wildcat so go get some and stay tuned for late week developments because we might have another snowfall coming our way.

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